High tension electrode for an electrical precipitator



HIGH TENSION ELECTRODE FOR AN ELECTRICAL PRECIPITATOR Filed 0ct. 28,1952

2 Sheets-Sheet 1 IN VEN TOR.

kw 4mm AT RNQEYS June 21, 1955 P. HERBER 2,711,224

HIGH TENSION ELECTRODE FOR AN ELECTRICAL PRECIPITATOR Filed Oct. 28, 1952 2 Sheets-Sheet 2 INVENTOR.

United States Patent HIGH TENSION ELECTRODE FoR AN ELEC- TRICAL PRECIPITATOR Application October 28, 1952, Serial No. 317,325 13 Claims. (Cl. 183-7) The present invention relates generally to electrical prccipitators, and more particularly to the' construction of the high tension electrode assembly and the individual discharge electrodes of that assembly.

It has been common practice to form the individual discharge electrodes from wire having a diameter of about A; inch, more or less. A plurality of such wires are arranged in parallel spaced relationship and mounted upon suitable frame members to form an electrode assembly. At these high tension electrodes, corona discharge is formed; and it is for this reason that wire is selected since this configuration of the electrodes facilitates the formation of corona discharge at each individual electrode. The individual electrodes, as well as the electrode assembly as a whole, are opposed by grounded electrodes which are usually signed in a manner to suppress the formation of corona. discharge at the grounded electrode and are therefore of the type generally referred to as a non-discharge electrode.

In industrial installations, it is common practice to apply rectified alternating current to these electrodes at voltages in the general range of 40,000 to 65,000 volts. The inherent electrical characteristics of an electrical precipitator, from both the standpoint of construction and operation, create in the applied current steep wave fronts. The steep fronts, combined with the high voltages, create within a discharge electrode assembly transient voltage differentials which at times may amount to several thousand volts over relatively short distances.

The result of this condition is that the individual wires are subjected to'sparking between them and contacting members wherever there is not a firm contact with the electrode. These small arcs, commonly referred to as spit arcs, form at many points where there is a small air gap between the wire electrode and another contacting or supporting member, even though both mem-' bers are part of the same electrode assembly- Formation of spit arcs is particularly acute where the electrode wire engages a supporting or guiding member or an attached shroud placed on the wire to suppress corona discharge locally. These spit arcs cause oxidation or electrical erosion of the electrodes; and when the arcing is continued over a suflicient time, an electrode is completely burned away at the location of the spit arc. Since it is normal practice to support. electrodesfrom their upper ends, and it may be at the support that the spit arcs occur, entire electrodes may drop out because they are burned through.

Many different expedients have been resorted to in order to obtain a firm bond or engagement between the electrodes and other members over the entire area designed to be in contact; but most such expedients have the objection that they are complicated and therefore expensive. While the item of cost on a single wire electrode is not great, any increase in cost becomes substantial in a large commercial installation because there may be several hundred such wire electrodes in asingle installation.

Thus it becomes a general object of my invention to devise a high tension discharge electrode assembly for an electrical precipitator which is free from the formation of spit arcs between the electrodes and other members of the assembly. 7

It is also an object of my invention to devise a discharge electrode which is designed and constructed in a simple, inexpensive manner that permits firm contact between. it and the supporting or guiding members that engage it, to eliminate spit arcs.

It is a further object of my invention toprovide a high tension electrode which may be simply and easily provided with a section at which corona discharge is suppressed in an eflicient manner and without creating a connection which is subject to the formation of spit arcs.

It is also an object of my invention to provide a discharge electrode well adapted over the major portion of its length to facilitate the formation of corona disice charge, but which can be shaped to provide an integral section at which corona discharge is substantially sup pressed.

These and other objects of my invention are attained in a high tension electrode assembly constructed according to my invention by providing a pair of vertically spaced frame members between which extend a plurality of thin, ribbon-like discharge electrodes. The several discharge electrodes are held spaced apart and parallel to each other by the frame members in order to receive. an opposing grounded electrode between two successive discharge electrodes. Each discharge electrode is a relatively thin, elongated ribbon-like metal strip having an intermediate section which normally constitutes the major portion of the length of the electrode and is adapted to facilitate the production of corona discharges at the thin edges of the strip. These electrodes are arranged in the assembly so that the thin edges of the discharge electrodes are directed toward opposing grounded electrodes and the broad faces of the electrodes face in the direction of gas flowing past the efectrodes.

The discharge electrode may be fastened to supporting or guiding frame members by means of a taper pin which passes through an opening in the frame member and firmly wedges a transversely curved section of the electrode between the pin and the frame member at the opening. The discharge electrode may also be provided with one or more relatively short sections intermediate its ends which are transversely curved in order to substantially suppress corona discharge where the electrode passes a grounded member. At these curved sections, the. strip is bent to a uniform radius over an arc ofat least about but by reducing the radius of curvature, the strip may be bent to extend over nearly a complete circle if desired.

How the above objects and advantages of my invention, as well as others not specifically referred to herein, areattained will be better understood by reference to the following description and to the annexed drawings, in which:

Fig. l is a vertical transverse section through a precipitator showing a high tension electrode assembly in position inside the electrical precipitator, viewed in the direction of gas flow;

Fig. 2 is an enlarged front elevation of a single discharge electrode;

Fig. 3 is a side elevation of the discharge electrode of Fig. 2;

Fig. 4 is an enlarged transverse cross section of an upper frame member of the electrode assembly showing theconnection between the discharge electrode and the frame member;

Fig. 5 is a further enlarged fragmentary plan view of the upper frame member of Fig. 4;

Fig. 6 is a transverse cross section through a discharge electrode on line 66 of Fig. 3;

Fig. 7 is a fragmentary side elevation of the lower portion of a modified form of electrode assembly showing discharge electrodes connected to a bottom longitudinal member; and

Fig. 8 is an enlarged vertical transverse section on line 88 of Fig. 7.

Referring now to the drawings, Fig. l is a vertical transverse section through an electricalrprecipitator containing discharge electrodes in a high tension electrode assembly constructed in accordance with my invention, the precipitator being constructed otherwise according to well known and conventional features of design. The precipitator includes a shell or housing indicated genherein disclosed. The electrode arrangement herein described is commonly referredto as a rod curtain-since the individual rods are spaced sufficiently closely in the direction of gas flow to have the same electrical erally at 10 which is here shown as being composed of a framework of structural steel members 10a over which is applied a covering 10b of sheet metal or the like to define a gas duct that encloses the stream of gas carrying suspended particles which are to be removed from the gas stream by the electrical precipitator. In a precipitator of the design shown, the direction of gas flow is horizontal in a direction perpendicular to the plane of Fig. 1. In accord with usual practice, the bottom portion of housing '10 is provided with hopper 11 into which the collected particles fall by gravity when the collecting electrodes are rapped. Material in the hopper is withdrawn by discharge means not shown in the drawings.

Housing 10, and more particularly the structural steel framework 10a of the housing, is also means for supporting in place one or more assemblies of high tension 1 discharge electrodes and an equal number of assemblies of opposing grounded electrodes. Between the discharge and grounded electrodes an electric field is maintained through which the gas stream and its suspended particles pass to separate or collect the particles. The action of the electrical field upon the suspended particles will not be described here since it is well known in the art.

One of the electrode assemblies supported by the shell is composed of grounded electrodes. The grounded electrodes are supported from a support frame consisting of a plurality of horizontal members 12, which extend transversely across housing 10 and are connected to the housing frame for support thereby, and a plurality of horizontal support members 14 which in turn are supported by members 12. From the support frame 7 there is suspended a plurality of grounded electrodes 15.

Individual electrodes 15 are here shown as being metal rods which extend vertically and are suitably connected at their ends to upper and lower longitudinally extending bars 14 and 16 respectively. A series of spaced rods 15 attached to one of the upper members 14 forms a v row of grounded electrodes that extend in a direction parallel to the direction of gas flow and perpendicular to the plane of Fig. 1. Upper longitudinal members 14 may be supported at their ends on two frame members i 12. Lower longitudinal members 16 are preferably not vertically supported in a manner to carry any of the weight of the electrodes but are received in slots in a transversely extending guide 18 which holds each mem: v

ber 16 against lateral movement. In this way a complete row of grounded electrodes 15 is maintained in a fixed position.

As may be seen from Fig. 1 there is typically a plurality of rows of electrodes 15, these rows being spaced tension applied by the weight.

apart across the width of the precipitator in a direction 1' transversely of the gas stream, while each of the row extends perpendicularly to this direction or parallel to the direction of gas flow.

It will be understood that my invention is not limited 15, and other electrodes may be used than the ones to any particular type or shape of grounded electrodes effect as if the electrode were a substantially solidor continuous member but with the advantage that the weight of a row of electrodes is substantially less than would be the weight of a plate or sheet of the same assembly. Each hanger rod extends upwardly through the top wall of housing 10 to pass upwardly through an electrical insulator 22 which is supported by a collar 24 at a point intermediate its ends. The collar rests upon a horizontal web which is part of insulator housing 23. is supported by the main precipitator housing 10. One hanger rod 21 of the group is ordinarily connected to high tension conductor 25 by means of which suitable high voltage current, from a source not shown, is supplied to the high tension electrode assembly.

Insulators 22 thus not only serve to insulate the high tension lead-in but also to insulate the various high tension members from the grounded shell of the precipitator.

In addition to hanger rods 21, the high tension electrode support frame includes a plurality of transverse beams 26 there typically being two such beams for an electrode assembly, one of them being shown in Fig. 1.

Beam 26 is supported by two hanger rods 21. Resting upon and supported by beams 26 are a plurality of longitudinal frame members 28 which are here shown as being hollow rectangular tubes, although members of other sizes and shapes may be used if desired. Members 28 extend parallel to the direction of gas flow and are spaced apart along the length of beams 26 transversely of the direction of gas flow. This particular construction of the support frame is preferred and typical of such structures as may be used, but such details as i the invention. V

Suspended from each of longitudinal members 28;

is a row of discharge electrodes 20, the row of electrodes .20 being positioned midway between two successive rows of grounded electrodes14. A similar row of high tension discharge electrodes 20 is located in each of the spaces between two rows of grounded electrodes so that the total number of discharge electrodes 'also may be considered as being arranged in a plurality. of rows which are spaced apart transversely of the direction of gas flow, each of the rows extending parallel to the gas flow and comprising a plurality of spaced individual discharge electrodes.

r As may be seen in Figs. 1 and 2 and will be described more in detail, there is fastened to the lower end of each individual electrode 20 a weight 30 which is prefer ably square in cross section. Each weight is slidably received in a guide ring 31. The weight is free to move vertically in the ring to allow for expansion and contraction in the length of the electrode under a constant The ring holds the weight against lateral movement in any direction so that the proper position of the bottom end of the electrode 'is maintained. The ring in cooperation with the high tension electrode support frame maintains the electrode 20 as a wholeparallel to the opposing grounded electrodes 14. For reasons which will become more apparent, it is preferable that rings 31 and weights 30 have a square or non-circular cross section so that each guide ring31 also prevents rotational movement of the weight within the ring about the longitudinal axis of the electrode.

Insulator housing 23 in turn rests upon and Guide rings 31 are connected to longitudinally extending bars 33 which in turn are mounted upon and supported by transverse bars 34, the bars forming an open gridwork which is held in place by a plurality, usually four, hanger rods 36. Hanger rods 36 are connected at their lower ends to grid bars 34 and at their upper ends to beams 26. Since hanger rods 36 are part of the high tension electrode assembly, they areat the same high potential as the individual electrodes and are therefore spaced from the grounded electrodes or other grounded elements. For this reason the hanger rods are preferably aligned with one row of discharge electrodes 20; and under some circumstances it may be found desirable to omit from such row a discharge electrode when for construction reasons it is desired to locate the hanger rod at or close to the position of the electrode.

The individual discharge electrodes 20 are shown in detail in Figs. 2 and 3. They are cut from lengths of a thin, ribbon-like metal strip, according to the length of electrode desired. It is preferable that the strip be relatively thin and be several times as wide as it is thick. Although a considerable range of dimensions for the electrode is entirely practical, a highly satisfactory electrode has been made from material which is .02 inch thick and A; inch wide having a width to thickness ratio of approximately 30:1. These dimensions are intended to be merely illustrative of a typical electrode and are not limitative upon the electrodes.

As shown particularly in Figs. 2,, 3 and 4, the upper end of the electrode has a terminal section 20a which is curved transversely. The radius of curvature is uniform and is chosen so that the length of the curve extends over approximately half a circle or an arc of about 180. This curved section is utilized in fastening the electrode to support member 28, as shown in Figs. 4 and 5. The curved section of the electrode is long enough to extend through frame member 28 and particularly through a circular opening 28a in the upper wall of the support member. The radius of this opening is substantially the radius of transversely curved section 20a of the electrode. A taper pin 38 is then inserted into opening 28a and driven down until the pin is wedged firmly in place. The upper end of the pin is larger than the diameter of opening 28a so that at one side the pin engages the wall of the opening in frame member 28 and at the diametrically opposite side is pressed firmly against one face of the discharge electrode. The other face of the electrode is pressed firmly against the side of opening 28a. Thus a very firm contact is maintained mechanically without the aid of other fastening means between the electrode and its immediate supporting member 28 and likewise between taper pin 38 and both the supporting member 28 and the electrode.

Of course the length of arc subtended by the transversely curved section 20a of the discharge electrode may difier from 180. If the electrode is bent into substantially a full circle, the radius of curvature is materially reduced and the size of pin 28 is correspondingly smaller. It becomes difiicult to insert so small a pin into a substantially cylindrical section of the electrode. Furthermore, the contact between the electrode, the pin, and the support tube 28 is now not as satisfactory. On the other hand, reducing the arc materially below 180 makes necessary an unduly large opening 28a which weakens the supporting member; and the corresponding increase in the diameter of the taper pin adds to the cost of the pin without securing any corresponding construction advantage. Balancing these considerations, it has been found that curving the electrode through approximately 180 of arc in the terminal section where it is attached to the supporting member 28 is a preferred construction.

For the major portion 20b of its length, each electrode 20 is a strip, preferably fiat, so oriented that the two thin edges at opposite sides of the strip are directed toward two opposing grounded electrodes 15. As is well known in the art, these thin edges are particularly adapted to facilitate the formation of corona discharge at the edges and therefore these edges are so. disposed that they are the portions of the electrode closest to the opposing grounded electrodes. As will also be seen from. Fig. l, a broad or wide face of each discharge electrode faces the gas stream, i. e., lies in a, plane transverse to the direction of gas flow. Hence, the broad faces of all the discharge electrodes in one longitudinal row of electrodes are parallel to each other. This orientation of each electrode, when once established, is maintained by the firm engagement of the electrode with its associated supporting member 28 and the resistance to rotation or lateral displacement of the bottom end of the electrode provided by guide. ring 31.

The spacing between a discharge electrode and an opposing grounded electrode is designed to give a satisfactory operation at a specified voltage and the ordinary operating conditions. It is obviously desirable to maintain this spacing as closely as possible; and this arrangement of electrodes is designed to achieve that end. It will be understood that the electrode is extremely resistant to horizontal bending or deflection in the direction of its greater dimensions because the full width of the strip, herein assumed to be inch, lends a marked degree of lateral stiffness to the electrode in this direction even though it is a relatively elongated member. Consequently, when oriented as described, the electrode has a very high degree of resistance to any bending forces tending to move any part of it closer to one of the opposing grounded electrodes. The spacing between the several discharge electrodes in a row extending longitudinally of the gas stream is not critical and minor changes in this spacing has no adverse effect on operation. Hence the electrodes are so disposed that their least horizontal dimension (thickness) is so oriented that their greatest deflection is toward another discharge electrode. The -tension applied to an electrode by weight 30 normally keeps it substantially straight and properly positioned; but there is an inevitable tendency for these electrodes to warp or bend somewhat when subjected to the comparatively high operating temperatures encountered in industrial precipitators and the pull of a Weight 30 may not be suflicient to eliminate any deflection in the electrode. Because the electrode is much wider than it is thick, a ratio of about 30:1, all of the horizontal deflection is substantially limited to direction of the lesser dimension with the result that ordinary warpage or changes in shape as a result of thermal conditions and the like are confined, because of the shape of the electrode, almost entirely to the direction of adjacent discharge electrodes and substantially without any change in the spacing between the discharge electrode andopposing grounded electrodes.

'By reference to Fig. 1 it will be seen that the high tension electrode support frame is disposed above the transverse members 12 and longitudinal members 14 supporting the grounded electrodes; and similarly the gridwork of bars 33 and 34 is disposed below guides 18 for the grounded electrodes. As a result, all discharge electrodes pass grounded members 14 and 16. Wherever a high tension member passes a grounded member in this manner it is desirable to have sufficient electrical clearance between the two members that there is no danger of an arc-over; and, especially in the case of a discharge electrode, it is desirable to form a portion of the discharge electrode in a manner that substantially suppresses the formation of corona discharge from a short length of the discharge electrode where it passes the grounded member. For this purpose, such electrode 20 is provided with one or more transversely curved secti'ons 200.2% a point intermediate its ends. 200 is an integral portion of the electrode that is curved arounda uniform radius to nearly a full circle. It is preferable to leave a gap of about inch between the extreme edges; of: the metal strip so that the are through which the electrode, of the width given above, extends Each section is about 325 or about .9 of a complete circle. This formation may be resorted to especially when it is desired to suppress to a maximum degree the tendency to form corona discharge at any poistion around the electrode, as when the discharge electrode is passing grounded members at more than one side of it.

A variational formation for the electrode at the intermediate section 200 is to curve the strip over at least about 180" of arc, in which case it resembles the formation described above for the section 29a. With this configuration, the electric field existing at each of the two thin edges of the strip acts upon the field at the other edge in a manner to greatly reduce or substantially eliminate the formation of corona discharge at these edges. This latter configuration is particularly effective if the thin edges of the strip are pointing away from the grounded member since the smooth rounded semicircular portion of the electrode, when presented to the opposing grounded member, does not facilitate the formation of corona discharge. The objective is to eliminate any tendency to form an are between the discharge electrode and the grounded member because corona discharge is eliminated or reduced below the arc-over point. This is accomplished here because of the shape of the electrode itself and without the addition or attachment of a separate shroud.

With the arrangement of electrodes and illustrated in Fig. 1, each discharge electrode 20 has two intermediate sections 200 which are designed to suppress the formation of corona discharge over the length of these portions of the electrodes. The upper one occurs where the electrodes pass grounded members 12 and 14 and the lower occurs where they pass grounded members 16 and 18. It is usual for the discharge electrodes to extend below the bottom of the grounded electrodes, thus passing grounded members at least once; but the grounded electrodes may or may not extend above the top ends of the discharge electrodes. Accordingly, the discharge electrodes usually have at least one such intermediate section 20c.

It will be seen that over the length of each section 200, the electrode itself has been deformed to a shape that suppresses corona discharge, and so may be referred to as a self-shrouded section. The flat ribbon there is curved to a U-shape or C-shape, or even to a full circle. The metal itself is unchanged in cross-sectional area, only the shape of the ribbon electrode is changed. Thus a shroud is formed integrally without the addition of any separate parts fastened to the corona forming member as heretofore been conventional practice.

Figs. 2 and 3 show in detail the connection between the strip electrode and the weight on the lower end. Each Weight has a slot 390 which has tapering side walls so that the mouth of the slot at the top of the weight is narrow than the inner end or base. A loop 20d formed on the lower end of the electrode is inserted from the side into slot 30a and a keeper pin 40 is then placed in the loop to spread it apart and cause the loop to engage the sides of the slot. Since the diameter of the keeper is greater than the opening at the mouth of the slot, keeper 40 prevents the loop from being withdrawn upwardly and secures the weight to the electrode. A clip 41 7 may be used to fasten the ends of the loop in place.

It is frequent practice to dispense with the weight on the end of each individual discharge electrode and connect electrodes at the lower end to a. rigid bar which may be part of a lower guide frame for positioning the electrodes. With this construction the frame as a whole serves to tension the electrodes and to hold them in by longitudinally extending members, 42 and transverse members 43.. The longitudinal members are preferably tubular or U-shaped as shown and are each directly beneath and parallel'to one of the top support frame members 28. They are interconnected by aplurality of transverse members 43 which may be angles or other A short length at the lower end of each ribbon discharge electrode is curved'in a plane transverse to the axis of the electrode to form the section 20 This section 20 extends through a circular opening in the upper wall of frame member 42, as shown particularly in Fig. 8. A taper pin 44 is then inserted in the opening in the frame member and driven home in order to tightly wedge the discharge electrode member against the wall of the opening and frame member 42. This attachment to the lower frame member 42 is essentially the same as described in more detail in connection with the at-. tachment of the electrode at its upper end to upper frame member 28. Otherwise the construction and de-. sign of the electrode assembly and of the individual high tension electrodes 20 is the same as described above.

Various directional terms, such as transverse and longitudinal have been used above referring to the length of the gas duct provided by housing 10 or the direction of gas flow therein. These terms have been used for descriptive purposes only to explain relative positioning of the parts and are not necessarily limitative upon the invention.

It will be apparent from the foregoing description I that various changes in the design and arrangement of parts of the discharge electrodes and electrode assemblies may occur to persons skilled in the art but which will not involve departure from the spirit and scope of my invention. Consequently, it is desired that the foregoing description be considered as illustrative of, rather than limitative upon, the appended claims.

I claim:

1. In an electrical precipitator for removing particles suspended in a stream of gas, the combination comprising: a grounded housing defining a gas duct; a pair of spaced grounded electrode supporting members supported on the housing; a pair of horizontally spaced vertically extending grounded electrodes suspended one from each of said supporting members; high tension electrode support means above said electrode supporting members; and a thin, ribbon-like discharge electrode suspended from the high tension support frame, said discharge electrode being positioned in the space between the grounded electrodes and oriented with its thin edges toward the grounded electrodes, said discharge electrode having an integrally formed, horizontally curved section adapted'to suppress the formation of corona discharge where the electrode passes the grounded electrode supporting member.

2. A high tension electrode assembly for an electrical precipitator removing suspended particles from a stream of gas, comprising: a pair of vertically spaced frame members; a plurality of thin, ribbon-like discharge electrodes extending between said frame members and spaced apart horizontally by said frame members, the discharge ing the curved section of the associated dicharge elec-.

lrodefirmly against the wall of the opening.

3. An electrode assembly as in claim 2 in which each discharge electrode has near its lower end a transversely curved section that passes through an opening in the lower one of the frame members; and a second plurality of taper pins, one pin passing through each said opening in the lower frame member and wedging the second memtioned curved section of the associated discharge electrode firmly against the wall of said opening.

4. An electrode assembly as in claim 2 in which each discharge electrode has intermediate and spaced from its ends a second integrally formed, transversely curved section which substantially suppresses corona discharge at said section.

5. An electrode assembly as in claim 4 in which each intermediate transversely curved section extends over an arc of at least about 180.

6. An electrode assembly as in claim 4 in which each intermediate transversely curved section conforms to an arc of substantally uniform radius extending for about .9 of a complete circle.

7. A discharge electrode for an electrical precipitator comprising a thin elongated ribbon-like metal strip having a flat intermediate section adapted to facilitate corona discharge at the edges of the strip, a transversely curved section adjacent one end, and an intermediate section also transversely curved to substantially suppress corona discharge at said section.

8. A discharge electrode as in claim 7 in which the curved section adjacent one end extends over an arc of at least about 180.

9. A discharge electrode as in claim 7 in which the curved section intermediate the end conforms to an arc of substantially uniform radius extending over about .9 of a complete circle.

10. A discharge electrode for an electrical precipitator comprising a thin ribbon-like metal strip having a flat intermediate section adapted to facilitate corona discharge at the edges of the strip and an integrally connected transversely curve section at which the strip is substantially uniformly curved between its longitudinal edges.

11. A high tension electrode assembly for an electrical precipitator removing suspendedparticles from a stream of gas, comprising: a pair of vertically spaced frame members; and a plurality of thin, ribbon-like discharge electrodes extending between said frame members and spaced apart horizontally by said frame members, the discharge electrodes all being arranged with their broad faces in substantially parallel planes, and each discharge electrode having an integral section intermediate and spaced from the ends of the electrode that is transversely curved to suppress corona discharge at the section.

12. An electrode assembly as in claim 11 in which each intermediate transversely curved section extends over an arc of at least about 13. An electrode assembly as in claim 11 in which each intermediate transversely curved section conforms to an arc of substantially uniform radius extending for about .9 of a complete circle.

. References Cited in the file of this patent FOREIGN PATENTS 534,472 Germany Sept. 26, 1931 

1. IN AN ELECTRICAL PRECIPITATOR FOR REMOVING PARTICLES SUSPENDED IN A STREAM OF GAS, THE COMBINATION COMPRISING: A GROUNDED HOUSING DEFINING A GAS DUCT; A PARI OF SPACED GROUNDED ELECTRODE SUPPORTING MEMBERS SUPPORTED ON THE HOUSING; A PAIR OF HORIZNTALLY SPACED VERTICALLY EXTENDING GROUNDED ELECTRODES SUSPENDED ONE FROM EACH OF SAID SUPPORTING MEMBERS; HIGH TENSION ELECTRODE SUPPORT MEANS ABOVE SAID ELECTRODE SUPPORTING MEMBERS; AND A THIN, RIBBON-LIKE DISCHARGE ELECTRODE SUSPENDED FROM THE HIGH TENSION SUPPORT FRAME, SAID DISCHARGE ELECTRODE BEING POSITIONED IN THE SPACE BETWEEN THE GROUNDED ELECTRODES AND ORIENTED WITH ITS THIN EDGES TOWARD THE GROUNDED ELECTRODES, SAID DISCHARGE ELECTRODE HAVING AN INTERGRALLY FORMED, HORIZONTALLY CURVED SECTION 